In undoped cuprates, Cu\(^{2+}\)  ions contain 9 electrons with partially
occupied \(3d_{x^{2}-y^{2}}\)  orbitals. The oxygen \(2p\)  orbitals are higher in
energy than the Cu \(3d_{x^{2}-y^{2}}\)  lower Hubbard band. Thus, cuprates
belong to the so-called charge-transfer insulator. A superexchange
interaction between localized Cu \(3d_{x^{2}-y^{2}}\)  spins is mediated by
oxygen ions and causes an antiferromagnetic (AF) ground state. Upon chemical
doping, holes may be introduced on the oxygen sites in the CuO\(_{2}\)  planes
[1]}, [2]}, [3]} and combine with the \(3d_{x^{2}-y^{2}}\) 
spins to form the Zhang-Rice singlets [4]}, destroying the
long-range AF order rapidly. In theory, these led to an effective \(t\) -\(J\)  model, describing the holes moving on the
antiferromagnetic square lattice. High temperature SC with robust \(d\) -wave
pairing has been predicted and established over a wide doping range [5]}, [6]}, [7]}. Extending such “cuprate-Mott"
conditions in other oxides has led to extensive efforts on nickel oxides
[8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}. Nickelate superconductors have a similar layered crystal structure with Ni\(^{1+}\)  possessing the same \(3d^{9}\)  configuration as Cu\(^{2+}\) . As a result,
theories based on Mott scenario have naturally been developed to account for
nicklate superconductors.
